Lanthanides anion binding

Dickins, R. S. Gunnlaugsson, T. Parker, D. Peacock, R. D. Reversible anion binding in aqueous solution at a cationic heptacoordinate lanthanide centre selective bicarbonate sensing by time-delayed luminescence. Chem. Commun. 1998, 16, 1643-1644. 

The angular and distance information provided by the lanthanide induced shift has found widespread application from the determination of solution structures of Ln chelates [18,19] to gaining structural information on proteins, nucleotides and amino acids [19], More recently anion binding to coordinatively unsaturated lanthanide complexes has been effectively signalled as the observed lanthanide induced shift has been directly correlated to the nature of the donor atom in the axial position [8,20,21], It is the polarisability of the axial donor that ranks the second order crystal field coefficient, B02, and hence determines the magnitude of the observed shift. Values of the mean shift of the four most-shifted axial protons of the 12-Nq ring for [Yb.la]3+ are collated in Table 2. 

The most thermodynamically stable and kinetically inert complexes of the trivalent lanthanides are those of the ligand DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) (42, 43). Our search for lanthanide macrocyclic complexes that would remain intact for longer time periods led us to examine derivatives of DOTA. There are two potential difficulties with the use of DOTA complexes of the trivalent lanthanides for RNA cleavage. First, the overall negative charge on the complex is not conducive to anion binding for example, Gd(DOTA)-does not bind hydroxide well (44). Second, DOTA complexes of the middle lanthanides Eu(III) and Gd(III) have only one available coordination site for catalysis. The previous lanthanide complexes that we used, e.g., Eu(L1)3+, were good catalysts and had at least two available coordination sites. 

Another lanthanide-based anion sensor 2 exploits a bipyridine phosphine oxide derivative as a pentadentate ligand to coordinate Eu cation. As anions bind to this complex, emission intensity was increased, presumably again due to the displacement of ligated water molecules. 

S. J. Butler and D. Parker, Anion Binding in Water at Lanthanide Centres From Structure and Selectivity to Signalling and Sensing, Chem. Soc. Rev., 2013, 42, 1652. 

Lipophilic lanthanide complexes of fluorinated 3-diketonate ligands were demonstrated to bind unprotected amino acids under neutral conditions. It is not clear whether amino acids are bound as anions or zwitterions. Chiral ligands 63-66 have been prepared and tested for extraction of amino acids from water into dichloromethane [84] (Table 7). NMR and CD spectroscopic... 

Further use of relaxation data, now studying the water and the ligand protons34 36, leads to an estimate of the outer sphere hydration of the lanthanides. We know there are no water molecules in the first coordination sphere of course. These outer sphere relaxation data for the different cations are proportional to susceptibilities and electron relaxation times and become very useful in the study of the inner sphere hydration of other complexes M(dipic) (H20)x and M(dipic)2(H20)y, see below. Note that there is no evidence of further association of the Ln(III) tris-dipicolinate complexes with small cations such as sodium ions. Later we shall show that these anions can bind to biological cationic surfaces and act as shift or relaxation probes. 

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